What equipment did Poseidon use for the first bathymetric measurement?

The correct choice is the sounding line, which is a traditional tool used in nautical navigation and hydrographic surveying to measure the depth of water. This method involves lowering a weighted line into the water until it touches the seabed. The length of the line that has been submerged indicates the depth, allowing for bathymetric measurements to be taken.

The sounding line was one of the earliest and simplest forms of bathymetric measurement, predating more advanced technologies. It highlights the innovative methods utilized by early navigators, including the legendary Poseidon, to map the underwater topography.

In contrast, drogues are devices used to measure current and track water movement, echo sounding relies on sonar technology to determine depth by emitting sound waves, and measuring tapes are not specifically designed for underwater depth measurement. Each of these methods is notable for its application in hydrographic surveys but does not represent the original technique for bathymetric measurement employed by ancient mariners. Therefore, the sounding line stands out as the fundamental tool used in early underwater depth assessments.